1. Field of the Invention
This invention relates to a multi-phase PSK (phase-shift-keying) demodulator, particularly adapted for use in multiplexed PSK transmissions of digital signals.
2. Description of the Prior Art
Conventional techniques for recovering a synchronized carrier wave from a PSK-modulated carrier wave for demodulation include reverse modulation, frequency multiplication, the Costas loop system, and the like.
In the reverse modulation technique, a received PSK-modulated carrier wave is demodulated with the output of a voltage-controlled oscillator (VCO) used as a reference, and the demodulation output is caused to reverse-modulate the PSK-modulated carrier wave so as to provide an unmodulated carrier wave for demodulation. The unmodulated carrier wave is phase-compared with the output of the VCO for the automatic frequency control thereof. Thus, the VCO output is kept synchronized with the PSK-modulated carrier wave, so that the synchronized VCO output is used as the synchronized carrier wave for demodulation.
Since such a system does not compensate for phase variations at the phase detector, the reverse modulator, and the like, the output waveform of the phase detector may deteriorate, thus increasing the error rate and adversely affecting the synchronization. Furthermore, with an increase in the number of prefixed signal phases available to PSK, the construction of the reverse demodulator becomes quite complicated.
In the frequency multiplication technique, the PSK-modulated carrier wave is frequency-multiplied by a number equal to the number of phases present. The frequency multiplying signal and the VCO output signal are phase-compared by a phase comparator, whose output is fed to the VCO to control the oscillation frequency. The phase-controlled VCO output is then frequency-divided, and the PSK-modulated carrier wave is synchronized with the frequency-divided output. According to this technique, phase variations at the frequency multiplier, frequency-divider, and the like, adversely affect synchronization. In addition, with an increase in the number of discrete phases, the frequency multiplier and frequency divider become increasingly complicated. Furthermore, these conventional techniques require the use of complicated signal processing circuitry in the high frequency band.
The Costas loop system is designed to avoid the aforesaid shortcomings. For details of its structure and function, reference is made to the Article entitled "Synchronous Communications", in the Proceedings of the IRE, VOL. 44, No. 12, pp. 1713-1718, December Edition, 1959. However, this system is not free from shortcomings in that it requires a stabilized DC amplifier and twice as many phase detectors as the above-mentioned techniques.
Among these three conventional techniques, reverse modulation is most often used because of its relative simplicity.